Elevator systems are widely used for transporting people or goods from one point to another. An elevator system typically includes an elevator car connected to a counterweight by hoisting ropes that move the elevator car vertically in an elevator shaft or hoistway. The hoisting ropes extend over a sheave or machine. A motor connected to the machine provides power to move the elevator car between two positions. The machine (and specifically the motor of the machine) is provided with a brake system to control the motion of the elevator car and to stop the elevator car as desired. The elevator car, motor, brake system, and other electrical components of the elevator system are supplied power via a power supply system connected at least indirectly to the elevator system.
Traditionally, the power supply system has been a transformer that receives a line voltage (such as from a power mains) which it converts to a voltage level suitable for use by the various components of the elevator system. Advantageously, the transformer is able to limit the output voltage that is supplied to the elevator system to a desired voltage level. However, the efficiency of the transformer often varies with the electrical load, going down, sometimes significantly, at standby operation of the elevator system. Further, transformers are often big, heavy, and costly to operate and maintain.
To circumvent at least some of the disadvantages of traditional transformers, in recent times, a switch mode power supply to power the elevator system has been used. A typical alternating current to direct current switch mode power supply may include a rectifier to convert incoming alternating current line voltage to direct current voltage, which may then be chopped or switched periodically using a power transistor to obtain a smooth direct current voltage signal. The direct current voltage signal may then be inverted back into an alternating current voltage signal to be passed through the windings of a transformer and a rectifier to obtain the desired direct current output voltage. Such a switch mode power supply has several advantages. For example, the switch mode power supply can provide higher efficiency even at lower electrical loads because the switching power transistor dissipates little power. Other advantages include smaller size and lighter weight and lower heat generation due to higher efficiency. However, unlike a traditional transformer, the output voltage from the switch mode power supply may be difficult to limit to a desired voltage level.
The International Electrotechnical Commission (IEC) regulates the maximum output voltage that a power supply component, such as the switch mode power supply can generate. Specifically, the IEC states that the output voltage from a power supply component cannot exceed the extra low voltage under normal conditions and under single-fault conditions in the power supply component. Thus, to protect from electric shock, the IEC requires that a power supply component provide a protective extra low voltage (PELV) by limiting the maximum output voltage to a ripple-free sixty volts of direct current (60 VDC) under dry conditions.
Accordingly, it would be beneficial if a switch mode power supply capable of providing a protective extra low voltage in accordance with the IEC standard could be developed.